Allow to setup custom storage

[zorn-v]

Currently I write middleware app and want to use vue-auth there. But problem is that it does not have (not persist) cookie/storage, but I can (and want) to store token by myself via expose some object (say QtApp) to js.
With this patch I can do something like this

const options = {
//...
  stores: [{get(key){}, set(key, token){}, remove(key){}}]
//...
}

[websanova]

Alright, this is up in 4.1.1, thx :-)

[zorn-v]

Thanks for the speed 😄